Ball screw drives are used in vehicles above all in steering gears. The purpose of the ball screw drive is in this case to convert a rotary movement of an electromotive drive into a translatory movement. A conversion of the rotary movement into a translatory movement is required in the steering gear in order to bring about a translatory movement of a tie rod.
In the prior art, this is described for example in DE 102006061949 A1 and EP 1553006 A1.
In DE 102006061949 A1, a rotational movement is generated by an electric servomotor. The rotational movement is supplied via a rotor shaft to a clutch and then directly to a gear. The rotor shaft is embodied as a hollow shaft. The built-in gear is in this case designed as a circulating ball gear. The rotational movement of the servomotor is converted by the circulating ball gear into a translatory movement of a ball screw spindle. The servomotor takes up quite a lot of space because it has to run, on account of the direct drive of the ball nut, slowly and with high torque. The servomotor is also relatively heavy; this has an adverse effect on the fuel consumption of the vehicle.
In EP 1553006 A1, the power for the translatory movement of the ball screw spindle is transmitted by a pulley, which is operated by a drive belt, to the ball screw spindle via a ball screw nut and balls. The drive power is in this case transmitted by an electric motor to the drive belt. This drive variant of the ball screw spindle has the drawback that the toothed belt is an expensive component on account of the stringent demands placed on its service life.
DE 3735517 C2 discloses an actuator for actuating a rear-axle steering system with a screw spindle, a hollow shaft motor and a reduction gear designed as a planetary gear. In this gear, the hollow wheel is connected to the ball nut, the planet carrier is secured to the housing and the sun wheel is connected to the motor shaft. This arrangement has a relatively long overall length and the ball nut is mounted at both ends with ball bearings.
DE 3836255 C2 discloses a similar actuator for rear-wheel steering with a hollow shaft motor, a circulating ball assembly and a planetary gear. The hollow shaft motor drives the sun wheel, the hollow wheel is secured to the housing and the planet carrier is connected to the ball nut. Complex mounting both of the ball nut and of the rotor in relation to the ball nut is required in this case too.
EP 1813506 A1 discloses an electric power steering system with a hollow shaft motor and planetary rolling gear. In this case, the planets are designed as elongate screw spindles. The sun wheel is the screw spindle of the toothed rack. A circulating ball assembly is not provided.
The object of the invention is therefore to provide a drive for the translatory movement of the ball screw spindle that takes up less space, has a lighter motor and has to produce a lower drive torque.
The object is achieved by a power steering system of claim 1; the further sub-claims specify expedient developments.